Molecular rearrangements Benzidine rearrangement

Scheme 13 may look unfavorable on the face of it, but in fact the second two reactions are thermally allowed 10- and 14-electron electrocyclic reactions, respectively. The aromatic character of the transition states for these reactions is the major reason why the benzidine rearrangement is so fast in the first place.261 The second bimolecular reaction is faster than the first rearrangement (bi-molecular kinetics were not observed) it is downhill energetically because the reaction products are all aromatic, and formation of three molecules from two overcomes the entropy factor involved in orienting the two species for reaction. 

Lob,3 convinced of the futility of thus being able to obtain a good yield of benzidine by a direct reduction of nitrobenzene in acid solution, sought to carry out the benzidine process by a careful realization of the conditions theoretically required— primary preparation of azoxy- or azobenzene in the best quantitative yields, i.e. in electrolytes, containing alkali or alkali-salt, then reducing these products in acid solution. Two processes thus resulted. In the first one the electrolytic reduction was carried out to azobenzene in alcoholic-alkaline solution, then the cathode solution was acidified with sulphuric acid, and the further reduction and molecular rearrangement combined in one operation. The second process, which was... 

The influence of the cathode metal is much more manifest when acid electrolytes are employed than in alkaline reduction. In alkaline solution at copper electrodes, if we except the last-mentioned process, the rapidly occurring condensation of the first reduction phases—of the nitroso- and hydroxylamine body—always leads immediately to the azoxy-body and makes this appear to be the typical product of the alkaline reduction, which can in turn be further reduced. In acid solution this condensation takes place so slowly that the molecular rearrangement of the hydroxylamine and its further reduction to amine has time to take place alongside the formation of the azoxy-body and the reduction of the latter to the hydrozo-compound or benzidine.4... 

This 4- 4-di-amino di-phenyl is benzidine which yields a very important group of dyes and which is formed by a molecular rearrangement from hydrazo benzene (p. 578). ... 

H. J. Shine, The Benzidine Rearrangement , in Mechanisms of Molecular Migrations (Ed. 

The scheme (122) belongs to the class of so-called cage-dissociation theories (Banthorpe et al., 1964) and is consistent with two well-established properties of the acid-catalysed benzidine rearrangement (i) its kinetic order in proton varies from 1 to 2 (ii) the absence of evidence for inter-molecularity. 

Benzidine rearrangement of hydrazobenzene is used to obtain 4,4 -diphenyldi-amines from nitrobenzenes. This reaction occurs intra-molecularly, by way of a p-quinonoid intermediate. 

Benzidine-like rearrangements are reported on acid treatment of suitable imidazoles , pyridine and thiazole hydrazocompounds , but not ferrocene derivatives. Related, probably intramolecular, rearrangements of/>-quinamines and 0,iV-diarylhydroxylamines , (reactions 106 and 107) have been arbitrarily assigned w-bond mechanisms, whereas reactions (108) and (109) are probably inter-molecular, although the observed formation of cross-over products counts for litde . ... 

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